1. Field of the Invention
The present invention relates to a means of a semiconductor module obtained by mounting a semiconductor element, e.g., a memory integrated circuit on a multilevel interconnection board, which achieves high heat dissipation of the semiconductor element.
2. Description of the Related Art
A memory module is an example of a semiconductor module in which a plurality of memory ICs are mounted on one multilevel interconnection board.
FIG. 1 shows a portion of a conventional memory module on which memory ICs are mounted. Referring to FIG. 1, reference numeral 81 denotes a multilevel interconnection board; 82, insulating layers of the multilevel interconnection board 81; 83, inner wiring layers of the multilevel interconnection board 81; 84, metal wiring layers (e.g., copper foils) on the surfaces of the outer layers of the multilevel interconnection board 81; 85, memory ICs mounted on the device mounting surfaces (e.g., upper and lower surfaces) of the multilevel interconnection board 81; 86, chips of the memory ICs; 87, molding resin packages of the memory ICs; 88, external leads of the memory ICs; and 89, electronic components (e.g., chip capacitors) other than memory ICs that are mounted on the device mounting surfaces of the multilevel interconnection board 81.
The inner wiring layers 83 supply a power supply potential Vcc or a ground potential Vss, and the metal wiring layers 84 transmit a signal. This structure facilities the design of signal wiring lines and the like of the multilevel interconnection board 81. Each inner wiring layer 83 has the same thickness (e.g., about 40 .mu.) as that of each metal wiring layer 84.
The external leads 88 of each memory IC are soldered to foot wiring portions connected to the corresponding memory IC. The leads of electronic components other than the memory ICs are soldered to the foot wiring portions on the device mounting surfaces to be connected to chip capacitors.
In the memory module described above, heat generation by the chips 86 of the memory ICs 85 gradually increases as the operation speeds of the corresponding memory ICs 85 are increased. Although each memory IC 85 contacts the metal wiring layer 84 on the surface of the outer layer of the multilevel interconnection board 81, a corresponding package 87 does not contact the metal wiring layer 84 on the surface of the outer layer of the multilevel interconnection board 81. Hence, the heat dissipation paths of each memory IC theoretically include a first path extending from the chip 86 through the package 87 to dissipate heat in air, and a second path extending from the chip 86 to dissipate heat in air through the external leads 88 and the metal wiring layer 84.
However, it is known that the second path described above is almost completely ineffective. This is because each metal wiring layer 84 is a copper foil having a thickness of about 40 .mu.m, and a resist (not shown) is usually covered on the surface of the metal wiring layer 84. Accordingly, in the conventional memory module, the heat dissipation path from the chip 86 of each memory IC 85 depends on only the first path extending from the chip 86 to dissipate heat through the package 87. Therefore, heat dissipation in the conventional memory IC 85 is not sufficient.